In recent years, in a cellular mobile communication system, it has been becoming a common practice to transmit not only sound data but also large-volume data such as static image data and moving image data along with implementation of multimedia information service. In order to realize large-volume data transmission, studies have been actively conducted on a technique for realizing a high transmission rate by utilizing a high frequency wireless bandwidth.
However, when a high frequency wireless bandwidth is utilized, a high transmission rate can be expected at a short distance but attenuation is increased in accordance with a transmission distance as the distance is increased. Hence, when a mobile communication system in which a high frequency wireless bandwidth is utilized is actually placed in operation, a coverage area of a wireless communication base station apparatus (hereinafter abbreviated as a “base station”) is reduced, and therefore, there arises the necessity for installation of a larger number of base stations. Since the cost of installation of base stations is correspondingly high, there is a strong demand for a technique for realizing communication service that utilizes a high frequency wireless bandwidth while suppressing an increase in the number of base stations.
To satisfy such a demand, studies have been conducted on a relay transmission technique in which a wireless communication relay station apparatus (hereinafter abbreviated as a “relay station”) is installed between a base station and a wireless communication terminal apparatus (hereinafter abbreviated as a “mobile station”) so as to increase a coverage area of each base station, and communication between the base station and the mobile station is performed via the relay station. FIG. 10 is a schematic diagram illustrating an overall configuration of a conventional wireless relay system. With the use of the relay technique illustrated in FIG. 10, a terminal (mobile station 20), which is unable to directly communicate with a base station 10, is also allowed to communicate with the base station 10 via a relay station 30. Note that a mobile station 21 is a terminal subordinate to the base station 10.
[Description of TDD TD Relay]
Further, as a method for dividing links into an uplink (UL) and a downlink (DL), a TDD system is known. In the TDD system, links are divided into an uplink (hereinafter referred to as “UL”) and a downlink (hereinafter referred to as “DL”) in a time-division manner. Referring now to FIG. 11, general outlines of a relay system in which a relay station is applied in a TDD system will be described below. FIG. 11 is a conceptual diagram of the relay system in which the TDD system is used for relaying of the relay station 30.
Hereinafter, for the sake of description, the base station 10, the relay station 30, the mobile station 21 and the mobile station 20 will be simply referred to as “eNB”, “RN”, “UE1” and “UE2”, respectively.
For example, when the term “TRANSMISSION UL” is provided in FIG. 11, a signal is transmitted via an uplink (UL) in the direction indicated by the arrow from any one of UE1, LTE-A UE2 and RN, which plays a predominant role in the corresponding operation (left end of FIG. 11), by using any one of subframes #2 to #5 serving as the corresponding subframe (upper row in FIG. 11).
As illustrated in FIG. 11, RN uses part of resources allocated to UL and part of resources allocated to DL to transmit/receive data to/from eNB, and during this period, RN suspends service provided to UE2 connected to RN. FIG. 11 illustrates an example in which the subframes #2 and #3 are UL subframes serving as subframes for the uplink, and subframes #4 and #5 are subframes for the downlink. In this example, the subframes #3 and #4 are used to perform communication between RN and eNB through UL and DL, respectively.